[R-390] Microphonic Parts?

[Barry]

Make that a 10M resistor.  It's the AGC line to the grid.

If I move my finger near it, the detector sees a significant increase in signal and I assume that's just line noise being picked up by the resistor and that's understandable.  Apparently the EF86 is a high-gain amp and it's doing what it should; however, if I replace that part with a new carbon resistor, things settle down quite a bit.  It will still pick up the AC noise from my finger, but the inherent noise on the detected waveform is significantly less.

Looking at the resistor, it appears to be a rather special looking unit, unlike the other carbon comps used in the other circuitry.  There are other 10M units that are plain old carbon comp.  Not sure why they would want such a different style resistor there.  I don't have a parts list for this model so I can't verify anything specific about it.  It does measure very close to 10M, though.

Thanks,
Barry - N4BUQ


 On Sat 14/04/12 12:25 PM , Barry n4buq at knology.net sent:
> I know that tubes can be microphonic.  Is it likely for passive components
> to be microphonic as well?
> I have an old Marconi LCR bridge that I'm trying to get working correctly
> again (it doesn't want to stay NULLed in R mode).  Whilst poking around at
> the input of the first amplifier (an EF86) with a small plastic stick and
> looking for bad grounds or solder joints, I noticed that a very light
> tapping on the components connected to the control grid causes quite a lot
> of noise at the detector (as observed on the scope).
> The components that seem to cause the most noise are a 1M resistor and a
> 470pF cap.  The cap is connected directly to the grid and the resistor is
> connected on the other end of the cap to ground.
> I've loosened and retightened the ground points at the tube base and gently
> pushed on the solder joints but nothing seems to produce this noise like
> the tapping does.  Tapping the tube itself produces a small bit of noise,
> but not nearly as much as these components.
> Is this "normal"?  I tend not to think so, but the control grid of the
> first amp is a pretty sensitive place to go knocking around on so I'm not
> sure if this might just be expected behavior.
> Not directly related to an R390 but I know you guys are a great source of
> help for things like this.
> Thanks,
> Barry - N4BUQ
> 
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